The present invention concerns an apparatus and a method for sending and receiving information in a cable TV network.
A cable TV network is a branched network in which a base station via an aerial receives TV signals from a satellite, or some other medium, and forwards the signals through a coaxial cable which branches to each individual user. The cable TV network is, second to the telephone network, the network that reaches the largest number of users. Therefore, it would be desirable co use the existing coaxial cable For new broadband services, such as the ordering of pay TV, interactive TV games, mail order services and so on. This requires the network to work in both directions so that the users can both receive information from the base station, called downstream signalling, and transmit information to the base station, called upstream signalling.
These cable TV networks were originally designed for one-way communication through a coaxial cable, but have developed from being merely a point-to-multipoint configuration, with the base station as the only transmitter, to comprising both shorter or longer sections of fiberoptic cable and several transmitters in the net, i.e. a multipoint-to-point configuration.
Choice of components and other dimensioning of the network have been influenced by the requirements that the communication should be one way, point-to-multipoint and inexpensive. With downstream data transmission the levels of noise and disturbance are reasonable and a known protocol may be used for the transmission. In the upstream direction the conditions are different, as the network is not designed to work in this direction. Noise from components and disturbances picked up by defective network elements are added from all branches of the network up to the base stations This problem is caused by the multipoint-to-point configuration and is called noise funnelling. In addition, there are more narrowband disturbances in the upstream band. Owing to this the same protocol cannot be used when transmitting upstream.
Digital information can be transmitted through the TV cable via a modem, which converts the information to a modulation of an analogue carrier wave, with a carrier frequency. Point-to-point configurations with transmission via modems are common in data communication and may even be used in cable TV networks. The disadvantage is however that in the base station one modem per user must be handled. With many users per base station this can become both an expensive and a time consuming solution.
When transmitting via a modem through the same cable, the transmissions must be multiplexed in time or frequency to avoid clashing in the cable, as described in xe2x80x9cCATV Return Path Characterization for Reliable Communicationsxe2x80x9d, Charles A. Eldering et al., IEEE Communications Magazine, August 1995. With time multiplexing the different transmitters can transmit during different time slots to the same receiver, and a multipoint-to-point configuration can be obtained. One problem with this is that each transmitter then uses the whole bandwidth of the utilized cable TV channel, and a narrowband disturbance somewhere in the band may destroy the entire transmission. Time multiplexing also leads to limited flexibility since each transmitter needs the whole frequency band and can only send during specific time slots. Each transmitter can also only be handled during a certain time period in the base station, which may cause problems if the number of transmitters is large. Even with conventional frequency multiplexing the high levels of disturbance in the upstream band cause problems at the receiver. Apart from this, each channel must be filtered out with a large number of filters, and it is difficult and costly to make these filters sufficiently narrowband. This means that the carrier frequencies cannot be placed too close to each other, which limits the number of carrier frequencies that can be used in the cable TV channel.
U.S. Pat. No. 5,225,902 describes a multiple carrier wave system, that is, a system for upstream signalling with multiple carrier waves, in the cable TV network. The problem of disturbances can then be solved through the base station monitoring the received signals and selecting a number of carrier frequencies which have the lowest disturbance level. These selected frequencies have no mutual correlation, but are selected on the basis of the current disturbance conditions: As the disturbance conditions vary over the day, the received signals are monitored continually and the frequency of the carrier wave is automatically changed when necessary.
The best carrier frequencies are communicated to the users, who then all send on the same, selected, best frequencies. In this way it is ensured that a number of carrier frequencies having the lowest disturbance level are always used, and the use of several carrier frequencies also makes the transmission more secure, as the probability that one of the signals reaching the recipient in a readable state increases. This method however requires some kind of time multiplexing so that the transmissions do not clash with each other, which would result in the problems described above.
U.S. Pat. No. 5,371,548 A and U.S. Pat. No. 5,425,050 both describe the use of OFDM within cable TV technology, but only for downstream signalling. It has been generally assumed that this solution would not work for upstream signalling. For example, EP 701 351 A2 discards this solution because the burst character of the upstream communication would make it too difficult to determine the phase of the received signals.
One problem when transmitting information in cable TV networks is the large number of modems needed if one modem is placed with each user, and one for each user in the base station. Admittedly, these modems can communicate through the TV cable according to various known protocols, but the result will be a point-to-point configuration, and, since the base station will hold a considerable number of modems, the solution will be expensive and lead to time-consuming handling of the many moderns.
With multipoint-to-point communication the base station must be able to determine which user is transmitting. For example frequency or time multiplexing may be used. Both types of multiplexing lead to a loss in transmission capacity, in time and in frequency, respectively. Time multiplexing gives limited flexibility and conventional frequency multiplexing requires a large number of very narrowband filters.
One object of the invention is to provide a quick, flexible and reliable transmission of information through cable TV networks with multipoint-to-point configuration in the upstream direction, i.e. in the direction towards the base station.
A further object of the invention is to overcome the problems associated with conventional frequency multiplexing, i.e. to reduce the number of filters needed in the receiver of the base station and to achieve a frequency multiplexed transmission with closely spaced carrier frequencies and thus a high degree of utilization of the frequency band used.
Another object of the invention is to compensate for differences in propagation time which may occur between signals from different users.
Further it is an object of the invention to achieve reduced sensitivity to disturbances affecting transmission.
The above mentioned objects are achieved in a method which has acquired the characteristics mentioned in claim 1. Further characteristics and developments of the invention and an apparatus for carrying out the method are stated in the other patent claims.
According to the invention an apparatus and a method for transmission and reception of information in cable TV networks are provided in which each user transmits on a separate carrier frequency. The carrier frequencies are mutually orthogonal, so that the contribution of each carrier wave to the next one equals zero, and the separate channels can be separated through computation of at least one common algorithm, e.g. at least one FFT (Fast Fourier Transform). This FFT decodes all transmitting channels simultaneously so that all users can transmit at the same time. This ensures a fast and reliable transmission in the multipoint-to-point configuration.
According to the invention, the sensitivity to disturbances caused by time multiplexing is reduced. The FFT also reduces the number of different filters that would be needed if each channel was to be filtered out individually, which is the case in conventional frequency multiplexing.
As the frequencies are orthogonal and do not influence each other during separation with the FFT, they may be placed very close to each other in the utilized frequency band.
Carrier frequencies with excessive disturbances may be rejected in the case of changed disturbance conditions, but the frequencies used always maintain their mutual correlation. This correlation is here described as the signals being orthogonal, but they could also have some other kind of correlation enabling a common separation of the different transmission channels with at least one common algorithm.
Further, the invention provides a compensation for differences in propagation time that may occur between signals from different transmitters.